Field of the Invention
The present invention relates to an air conditioning apparatus in which at least one outdoor unit and a plurality of indoor units are alternately connected by refrigerant pipes.
Related Art
Conventionally, an air conditioning apparatus has been proposed in which at least one outdoor unit and a plurality of indoor units are alternately connected by a plurality of refrigerant pipes. If the temperature of the outdoor heat exchanger becomes equal to or lower than 0 degrees C. while this air conditioning apparatus is performing a heating operation, there is a possibility that frost forms on the outdoor heat exchanger. If frost adheres to the outdoor heat exchanger, the heat exchange between the refrigerant and outside air is hindered by the frost, so that there is a possibility that the heat exchange efficiency at the outdoor heat exchanger is reduced. Therefore, when frost forms on the outdoor heat exchanger, it is necessary to perform a defrosting operation to remove the frost from the outdoor heat exchanger.
For example, in an air conditioning apparatus described in JP-A-2009-228928 (page 9, FIG. 1), one outdoor unit having a compressor, a four-way valve, an outdoor heat exchanger and an outdoor fan and two indoor units each having an indoor heat exchanger and an indoor fan are connected by a plurality of refrigerant pipes. When the defrosting operation is performed while the heating operation is being performed by this air conditioning apparatus, the rotations of the outdoor fan and the indoor fan are stopped, the compressor is temporarily stopped, the four-way valve is switched so that the state of the outdoor heat exchanger is changed from a state of functioning as an evaporator to a state of functioning as a condenser, and the compressor is started again. By causing the outdoor heat exchanger to function as a condenser, the high-temperature refrigerant discharged from the compressor flows into the outdoor heat exchanger to thaw the frost adhering to the outdoor heat exchanger. Thereby, the outdoor heat exchanger can be defrosted.
As the condition for the shift from the heating operation to the defrosting operation, the following condition is preset: a condition where it is considered that frost forms on the outdoor heat exchanger such as when the state in which the temperature of the heat exchanger is equal to or lower than 0 degrees C. continues for 10 minutes or longer while the air conditioning apparatus is performing the heating operation (hereinafter, referred to as defrosting operation start condition), and when the defrosting operation start condition is satisfied, a shift from the heating operation to the defrosting operation is made. As the condition for ending the defrosting operation, the following condition is preset: a condition where it is considered that the frost adhering to the outdoor heat exchanger is thawed such as when the temperature of the outdoor heat exchanger becomes equal to or higher than 5 degrees C. (hereinafter, referred to as defrosting operation end condition), and when the defrosting operation end condition is satisfied, the heating operation is resumed from the defrosting operation.
On the other hand, when the heating operation is being performed by the above-described air conditioning apparatus, there is a possibility that the refrigerant oil discharged from the compressor together with the refrigerant accumulates in the refrigerant circuit of the air conditioning apparatus, so that there is a possibility that the amount of refrigerant oil in the compressor is reduced to cause lubrication deficiency in the mechanical part of the compressor. Therefore, when the air conditioning apparatus is performing the heating operation, it is necessary to periodically perform an oil recovery operation to return the refrigerant oil to the compressor.
When the oil recovery operation is performed, the rotation of the indoor fan is stopped, and as when the defrosting operation is performed, the compressor is temporarily stopped, the four-way valve is switched so that the state of the outdoor heat exchanger is changed from the state of functioning as an evaporator to the state of functioning as a condenser, and the compressor is started again. By driving the compressor with the refrigerant circuit in such a state, a refrigerant of high wetness flows through the refrigerant circuit, so that the refrigerant oil remaining in the refrigerant circuit is sucked into the compressor to be returned into the compressor.
As the condition for the shift to the oil recovery operation, the following condition is preset: a condition where the refrigerant oil is discharged from the compressor and the amount of refrigerant oil in the compressor becomes equal to or lower than an amount that hinders the operation of the compressor such as every time the total operating time of the compressor becomes three hours (hereinafter, referred to as oil recovery operation start condition), and when the oil recovery operation start condition is satisfied, a shift from the heating operation to the oil recovery operation is made. As the condition for ending the oil recovery operation, the following condition is preset: a condition where it is considered that a wet refrigerant (a condition where fluid refrigerant is contained in gas refrigerant) is sucked in the compressor and the refrigerant oil remaining in the refrigerant circuit is sucked into the the compressor together with the wet refrigerant such as when the superheating degree of the refrigerant sucked into the compressor (hereinafter, referred to as sucking superheating degree) becomes equal to or lower than 0 degrees C. (hereinafter, referred to as oil recovery operation end condition), and when the oil recovery operation end condition is satisfied, the heating operation is resumed from the oil recovery operation.